Hydraulic vehicle braking systems which are designed as power-operated braking systems and, in addition to a muscle-powered brake master cylinder, to which wheel brakes are hydraulically connected and which supplies pressure and volume for the actuation of wheel brakes, may include a further, electrically controllable pressure and volume supplying device, which controls the wheel brakes in a “brake by wire” operating mode, are known. In such a system, if the electrically controllable pressure and volume supplying device fails, the wheel brakes are actuated exclusively by the muscle power of the vehicle driver (unassisted fallback operating mode).
WO 2011/029812 A1 discloses an electrohydraulic braking system having a brake-pedal-actuated brake master cylinder, a travel simulator and a pressure supplying device. In a “brake by wire” operating mode, the wheel brakes are supplied with pressure by the pressure supplying device. In the fallback operating mode, the wheel brakes are supplied by means of the brake-pedal-actuated brake master cylinder with the pressure applied by the driver.
DE 10 2011 083 237 A1 discloses a braking system of the type described above. The braking system includes a hydraulic actuating device which can be actuated by means of a brake pedal and to which wheel brakes are connected, and a hydraulic travel simulator having a hydraulic chamber. The actuating device is hydraulically connected to the travel simulator via a hydraulic connection, wherein the hydraulic connection is hydraulically connected to a pressure medium reservoir by means of a normally open isolation valve. To modify the force/displacement characteristic curve of the brake pedal actuation, i.e. to modify the brake pedal feel perceived by the vehicle driver, pressure medium is discharged into the pressure medium reservoir via the isolation valve at the beginning of the braking operation, when the brake pedal travel is less than a predetermined limiting value. As soon as the brake pedal travel is greater than the limiting value, the isolation valve is closed. In the braking system described above, the isolation valve should not remain open for too long since, otherwise, the brake pedal “sags”, something that the vehicle driver will perceive as disturbing or irritating. However, activation for closure of the isolation valve is associated with the generation of noise, which can likewise be perceived as disturbing. In the case of a power failure, when the isolation valve cannot be closed, or when the isolation valve is leaky, the pressure medium volume displaced from the actuating device can escape into the pressure medium reservoir when the brake pedal is actuated in the previously known braking system, with the result that a pressure buildup in the wheel brakes is not possible or is possible only to a limited extent in the fallback operating mode, for example. This safety-critical situation should be avoided.
It is therefore an object of the present invention to provide a braking system having a variable force/displacement characteristic curve of the brake pedal actuation and a method for the operation of a braking system of this kind which ensure higher operational safety than the prior art in as economical a manner as possible. It should also be possible to operate the braking system with as little noise as possible.
According to the invention, this object is achieved by a braking system and method as described herein.